1. Field of the Invention
The present invention relates to audio visual equipment utilizing a visual image projector operating in synchronism with control signals and audio program signals recorded on magnetic tape. More specifically, the invention relates to a method for regaining synchronization between the visual image projector and the audio program recorded on magnetic tape.
2. Description of the Prior Art
A species of audio-visual equipment intended for educational use utilizes a compact cassette magnetic tape playback machine operating in synchronism with a visual image projector such as a slide or filmstrip projector. Typically, two types of signals are recorded on the magnetic tape. Audio signals are recorded to provide an audio instruction program to accompany the visual presentation, the cue tone bursts are recorded to provide control signals to advance a slide or filmstrip frame at an appropriate point in the audio instruction program. Cue tone control signals may also be provided for other instructional purposes.
Visual presentation is defined as the visual image projected by a projector from a film media (i.e. slide or filmstrip) and visual advance is defined as the change from one visual presentation to another. Visual advance may be initiated by manual operation of the projector controls or automatically thru means responsive to the recorded cue tones control signals.
The recording of the audio instruction program and the cue tone control signals may be done in accordance with either a super-imposed system or a separate track system as specified in American National Standards Institute (ANSI) specification P.N. 7.4-1975.
The super-imposed system provides for the recording of low frequency cue tone bursts on the same track as the audio instruction program. The cue tone burst may be recorded simultaneously with, and/or in sequential relation to the audio information signal. In the case of simultaneous super-imposed recording, the cue tone burst is recorded directly over the audio information signal. In the case of sequential super-imposed recording, the audio information signal is recorded between adjacent cue tone bursts. As used herein, the terms super-imposed, and mixed denote either simultaneous or sequential super-imposed recording. The cue tones consist of 50 Hz .+-. 5% tone burst. The burst time duration is 0.45 .+-. 0.07 seconds for visual advance and 2.00 .+-. 0.25 seconds for visual advance and program stop. During playback the magnetic tape transport means transports the magnetic tape past the sensing surface of a playback head. The playback head picks up both the audio instruction program signals and the cue tone control signals for reproduction. In order to prevent the lower frequency portion of the audio instruction program signals from being misinterpreted as a cue tone signal and causing a spurious visual advance, the output of the playback head is passed thru one or more frequency responsive filters to effectively separate the audio signals from the control signals. As shown in FIG. 1, the filter (s) may be a high pass filter having a transition frequency of 125 Hz and a minimum attenuation rate of 24 db/octave. The audio program is then presented to the student thru a loudspeaker or earphone and the cue tone control signals are used to provide visual advanced synchronized with the audio program.
The separate track system records the audio program on a first track and the cue tone on a second track. A 1,000 Hz cue tone is provided for visual advance and a 150 Hz cue tone is provided to stop the program. In addition, a 400 Hz and a 2300 Hz cue tone may be provided for unassigned control purposes. A first reproduce head picks up the audio signals from the first track and a second reproduce head picks up the control signals from the second track. Frequency selective filters then separate the various cue tones to perform their intended control functions.
The superimposed and separate track systems both have advantages and disadvantages. The superimposed system allows for the maximum recorded audio instruction program time for each compact cassette but the frequency restricted cue tone limits the number of control functions. Also, it is very difficult to rearrange the cue tones once the superimposed audio program and cue tone signals have been recorded. The separate track system allows a larger number of control functions and permits convenient erasing and rearranging of the cue tones. However, the use of a separate track system reduces the amount of audio program material that can be recorded to one half that of the superimposed systems.
A draw back of existing audio visual equipment using the superimposed system is that it is difficult to maintain audio visual synchronization when returning to a prior point in the audio visual program, or when advancing to a subsequent point in the program.
The following two examples illustrate these drawbacks.
1. A student desires to return to a prior point in the audio-visual program to review material. Using the manual control provided on the projector the student decrements the visual presentation in the reverse direction until the desired visual presentation is reached. The tape transport is then rewound on a trial and error basis until the appropriate point on the audio program is located. The synchronized audio-visual program is then resummed.
2. A student desires to advance to a subsequent point in the audio-visual presentation to avoid material previously learned. Using the manual control provided on the projector, the student increments the visual presentation in the forward direction until the desired visual presentation is reached. The tape transport is then fast forwarded on a trail and error basis until the appropriate point in the audio program is located. The synchronized audio-visual program is then resummed.
As is readily apparent, any departure from the predetermined audio-visual program requires the student to fast forward or rewind on a trial and error basis to locate the appropriate point in the audio program. It would be very desirable to provide means for automatically retaining or regaining audio-visual synchronization during or after rapid manual incrementing of the visual program in a forward direction or decrementing of the visual program in a reverse direction.
Audio-visual synchronization can be automatically regained if slide or filmstrip frame changes are counted as they are manually incremented or decremented, and the cue tones counted during the fast forward or rewind. When the two counts are equal, the tape transport can be stopped and the synchronized audio-visual program resummed. Counter means can be readily provided which increment one unit for each slide or filmstrip change in a forward direction or decrement one unit for each slide or filmstrip change in a reverse direction. As a practical matter, it has proven difficult, if not impossible, to accurately count the cue tones during fast forward or rewind. Conventional tape transports for compact cassettes provide a relatively constant angular velocity to the take up reel spindle. As the tape winds onto the take up reel, the effective radius of the take up reel increases. The linear velocity or speed of the tape being transported past the playback head is a function of the effective radius of the take up reel. During an end to end rewind or fast forward of a conventional compact cassette the linear tape velocity past the reproduce head can vary from approximately six to twenty-four times the normal tape playing speed of 1.875 in/sec (4.76 cm/sec). Thus the linear tape velocity can vary from a low of 11.25 in/sec (28.56 cm/sec) to a high of 45 in/sec (114.24 cm/sec). The frequency of the voltage induced into the playback head is directly proportional to the linear tape velocity past the head. During end-to-end rewind or fast forward of a conventional compact cassette the recorded 50 Hz cue tones can induce a signal ranging in frequency from 300 Hz to 1,200 Hz, and the low frequency portion of the audio program (125 Hz) can induce a playback signal ranging from 750 Hz to 3,000 Hz. As shown in FIG. 2, induced voltage frequency ranges of the cue tones and the audio program overlap in region "A". The 50 Hz filters used to separate the superimposed cue tones and audio program during normal playback can not be used during rewind or fast forward. A pre-set filter for the range of cue tone frequencies (FIG. 2, "B") cannot be used since the audio program material overlapping in region "A" can pass through the filter and be misinterpreted as a cue tone to cause a spurious visual advance count.
Applicant is aware of at least one attempt to accurately count the superimposed cue tones during fast forward (but not rewind). A conventional compact cassette tape transport is modified to provide capstan controlled fast forward. The capstan drive assures a constant linear tape velocity past the playback head, and induced cue tone and audio program frequencies in well defined, mutually exclusive ranges. A preset filter is used to separate the cue tone signals from the audio program signals. However, fast forward velocities greater than three times the normal playback velocity have been difficult to achieve. Also, mass produced compact cassette tape transports cannot be readily modified to provide capstan controlled rewind.
Applicant's invention provides a novel means of accurately separating the cue tones from the audio program material during fast forward and rewind in such a manner that the audio program material will not cause a spurious cue tone count and without modification of the conventional tape transport drive mechanism.